If you change your plan before the cut off date, your new plan will start on 2nd of next month. Otherwise, it will start on the 2nd of the following month.
For Example:
In a case where plan change cut off date is 24th Jan.
-Plan change requests submitted before and on 24th Jan will be activated by 2nd Feb.
-Plan change requests made on the 25th Jan and after will be activated by 2nd Mar.